tbf: Add BTS::ms_alloc method

Currently the code that creates the MS objects with tbf.cpp is
duplicated.

This commit moves the corresponding code into a new method. Since
there is no TLLI available there, the GprsMsStorage::create_ms method
has been refactored into two variants: one with TLLI/direction and
one without.

Sponsored-by: On-Waves ehf
diff --git a/src/gprs_ms_storage.h b/src/gprs_ms_storage.h
index ad1e656..df788bf 100644
--- a/src/gprs_ms_storage.h
+++ b/src/gprs_ms_storage.h
@@ -38,6 +38,7 @@
 
 	GprsMs *get_ms(uint32_t tlli, uint32_t old_tlli = 0, const char *imsi = 0) const;
 	GprsMs *create_ms(uint32_t tlli, enum gprs_rlcmac_tbf_direction dir);
+	GprsMs *create_ms();
 
 	const LListHead<GprsMs>& ms_list() const {return m_list;}
 private: